Mirror Modifier is visible in object mode but disappears in edit mode
You need to enable &quot;Display modifier in edit mode&quot;. This icon is of a box with four orange corners, and can be found on the modifier node with the other visibility settings.


Box
Displays the modified geometry in edit mode, as well as the original geometry which you can edit.

Without:

And with:

If you enable the Triangle option next to the box one you can then also edit the transformation caused by the modifier on edit mode


Triangle
When enabled, the final modified geometry will be shown in edit mode and can be edited directly.

For a detailed description on the use of these options read this page of the Blender manual.